A host computer system for virtual machines, such as the ESX® server available from VMware Inc. of Palo Alto, Calif., is able to support concurrent execution of multiple virtual machines (VMs). A guest operating system (OS) and applications run in the VM's memory and make demands on the physical memory of the ESX® server. Often, a VM may be configured with a large amount of virtual memory to handle unexpected increases in load. This, however, creates an increased demand on the underlying physical memory and limits the available physical memory for supporting other VMs hosted by the same server. If the resulting memory shortage is not managed well, the performance of the other VMs may degrade, even though increases in load do not materialize for the VM with the large memory allocation and the extra memory that has been allocated thereto is unused.